This application is based upon application No. 2002-240864 filed in Japan, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a drive mechanism employing an electromechanical transducer and a drive method therefor. More specifically, the present invention relates to the drive mechanism employing the electromechanical transducer such as a piezoelectric element and the like, suitable for driving a lens of a camera, a precision stage and the like, and the method for controlling the drive mechanism.
2. Description of the Related Arts
Conventionally, there has been proposed a drive mechanism employing an electromechanical transducer.
For example, FIGS. 1A through 1C show such a drive mechanism of an element-fixed type schematically. One end of a piezoelectric element in an extending and contracting direction is connected to a fixed member (or a stationary member) . The other end of the piezoelectric element in the same direction is connected to a drive member. The drive member moves in forward direction and backward direction, when the piezoelectric element extends or contracts. A moving body (or a driven member) is engaged with the drive member by a frictional force.
The moving body is driven when a voltage is applied to the piezoelectric element so as to cause the piezoelectric element to extend at a first velocity, and to contract at a second velocity, different from the first velocity. FIGS. 1A, 1B and 1C show respective states at points of time indicated by the reference characters A, B and C in FIG. 1D.
When the voltage gradually increases during a section A-B as shown in FIG. 1D, the piezoelectric element relatively slowly extends so that the state as shown in FIG. 1A changes into the state as shown in FIG. 1B. At the time, the moving body slides little, or it does not slide with respect to the drive member, and therefore the moving body moves together with the drive member substantially.
Secondly, when the voltage abruptly decreases during a section B-C as shown in FIG. 1D, the piezoelectric element contracts relatively fast so that the drive member returns to an initial position. At the time, the moving body stays at the present position substantially, and the moving body slides with respect to the drive member. Therefore, only the drive member returns to the initial position. As the result, the moving body moves from the initial position as shown in FIG. 1A to a forward position as shown in FIG. 1C.
Repeating such a cycle, the moving body moves along the drive member in a forward direction.
In the mean time, the moving body moves in a backward direction, when a voltage applied to the piezoelectric element has a waveform including rapidly increasing parts and gradually decreasing parts.
In order to run the drive mechanism, there are two manners of applying a voltage having a saw-tooth waveform to the piezoelectric element.
FIGS. 2 shows a first manner thereof. As shown in FIG. 2A, a waveform generator W, specifically a digital-analog transducer therein, for example, of 8 bits and 1-5 volts type, generates a voltage having a saw-tooth waveform. The voltage having the saw-tooth waveform is amplified, for example, up to 1-10 volts, by an amplifier M, and then is applied to a piezoelectric element X in order to run the drive mechanism. By adjusting the waveform generator W, a waveform of forward direction as shown in FIG. 2B and a waveform of backward direction as shown in FIG. 2C can be generated.
FIGS. 3 and 4 show a second manner thereof. FIG. 3 shows a circuit for applying a power-supply voltage V to a piezoelectric element X. The circuit includes constant current circuits A, D and switching circuits B, C. The waveform of forward direction or the waveform of backward direction are generated by actuating the constant current circuit A and the switching circuit B alternately, or by actuating the constant current circuit D and the switching circuit C alternately.
For example, the circuit is constituted as shown in FIG. 4A. When control signals are input to terminals xe2x80x9caxe2x80x9d, xe2x80x9cbxe2x80x9d, xe2x80x9ccxe2x80x9d and xe2x80x9cdxe2x80x9d of the circuit, the waveform of forward direction or the waveform of backward direction is generated, as shown in FIG. 4B.
Specifically, when the terminal xe2x80x9caxe2x80x9d is supplied with Hi input, the voltage applied to a piezoelectric element X gradually increases through the constant current circuit A as shown by the reference numeral 10 in FIG. 3B. Next, when the terminal xe2x80x9cbxe2x80x9d is supplied with Hi input, the piezoelectric element X is grounded through the switch circuit B, so that the voltage applied to the piezoelectric element X rapidly decreases as shown by the reference numeral 12 in FIG. 4B. Thus, the waveform of forward direction is generated.
In the mean time, when the terminal xe2x80x9ccxe2x80x9d is supplied with Hi input, the piezoelectric element X is connected to the power-supply voltage V through the switch circuit C, so that the voltage applied to the piezoelectric element X rapidly increases as shown by the reference numeral 14 in FIG. 4B. Then, when the terminal xe2x80x9cdxe2x80x9d is supplied with Hi input, the voltage applied to the piezoelectric element X gradually decreases through the constant current circuit D as shown by the reference numeral 16 in FIG. 4B. Thus, the waveform of backward direction is generated.
In the first manner, the waveform generator W and the power amplifier M are needed. In the second manner, the constant current circuits A, D and the switch circuits B, C are needed. Thus, the construction of the circuit is complex and leads to high cost. Moreover, it is difficult to achieve stable low-speed running of the drive mechanism.
Accordingly, it is an object of the present invention to provide a drive mechanism which comprises a drive circuit employing a simple construction and achieving stable low-speed driving.
It is another object of the present invention to provide a drive method for carrying out the stable low-speed driving.
In order to achieve one of the above objects, according to one aspect of the present invention, there is provided a drive mechanism, comprising: an electromechanical transducer having a pair of ends in expanding and contracting direction; a drive member fixed to one of the ends of the electromechanical transducer; a driven member which contacts frictionally with the drive member under a predetermined frictional force exerted therebetween; and a drive circuit for applying a drive voltage to the electromechanical transducer, wherein the drive voltage repeats a cycle of a first voltage having a value which is one of a maximum value and a minimum value, a second voltage having a value which is a value intermediate between the maximum value and the minimum value, and a third voltage having a value which is the other of the maximum value and the minimum value, and wherein applying the drive voltage to the electromechanical transducer causes the electromechanical transducer to expand and to contract so as to move the driven member along the drive member relatively.
Generally, the electromechanical transducer (for example, piezoelectric transducer, electrostriction transducer, magnetostriction transducer, electrostatic actuator and the like) changes the electrical energy (for example, electric voltage, electric current, electric field, magnetic field, static electricity and the like) supplied thereto into the mechanical energy (for example, transformation or strain such as prolonging, compressing, expanding, contracting, bending, twisting and the like).
In the configuration, while the drive voltage changes from the first voltage to the third voltage through the second voltage, the change of the drive voltage is relatively gradual. Therefore, the electromechanical transducer expands or contracts gradually so that the driven member moves together with the drive member without sliding with respect to the drive member substantially. In the meantime, while the drive voltage changes from the third voltage to the first voltage with maximum amplitude thereof, the change of the drive voltage is relatively rapid. Therefore, the electromechanical transducer expands or contracts rapidly so that the driven member slides with respect to the drive member. By repeating such a cycle, it is possible to move or drive the driven member with respect to the drive member in a predetermined direction.
According to the configuration, even if a period for which the second voltage is applied to the electromechanical element varies, variation of sliding amount per cycle caused by changing the drive voltage from the third voltage to the first voltage is little or nothing, and only the frequency of sliding, or interval period of the cycle, changes. Thereby, it is possible to run the drive mechanism at a low speed stably, by extending the period for which the second voltage is applied to the electromechanical transducer. Moreover, it is possible to realize the drive circuit for switching the drive voltage as described above, employing a simple construction.
It is not necessary to keep the second voltage constant, because small variation of the second voltage with cycles is not critical in connection with the performance of the drive mechanism. The time for which the drive voltage applied to the electromechanical transducer changes from the third voltage to the first voltage should be short enough to cause the relative sliding between the drive member and the driven member. Therefore , if such a criteria is satisfied, the drive voltage may keep, for example, 0 (zero) volt in the middle of changing from the third voltage to the first voltage.
As an embodiment, inequalities of
(n1+1)Txe2x88x920.3Txe2x89xa6taxe2x89xa6(n1+1)T+0.3Txe2x80x83xe2x80x83(1)
0.95(n2+0.5)Txe2x88x920.3Txe2x89xa6tbxe2x89xa60.95(n2+0.5)T+0.3Txe2x80x83xe2x80x83(2)
ta+tb less than Tdxe2x80x83xe2x80x83(3)
are satisfied, where T is a resonance period of a system including the electromechanical transducer and the drive member, where Td is a drive period of the drive voltage applied to the electromechanical transducer by the drive circuit, where ta is a first period for which the first voltage is applied to the electromechanical element, where tb is a third period for which the third voltage is applied to the electromechanical transducer, where n1 is one of zero and positive integers, and where n2 is one of zero and positive integers.
When the first period for which the first voltage is applied to the electromechanical element and the third period for which the third voltage is applied to the electromechanical transducer have some relationship with the resonance period of the system including the electromechanical transducer and the drive member as described in the above embodiment, it is possible to run the drive mechanism efficiently.
As an embodiment, the first voltage and the third voltage are equal and of opposite sign, and wherein the second voltage is zero volt.
In the embodiment, it is possible to reduce variation or kinds of parts of the drive circuit in order to employ a simple construction.
As an embodiment, applying the drive voltage to the electromechanical transducer causes the electromechanical transducer to expand at a first velocity and to contract at a second velocity, different from the first velocity, so as to move the driven member along the drive member relatively.
As an embodiment, the drive voltage applied to the electromechanical transducer has a generally square waveform comprising horizontal straight parts and vertical straight parts.
As an embodiment, the drive circuit comprises: a first switching unit for applying the first voltage to the electromechanical transducer; a second switching unit for applying the second voltage to the electromechanical transducer; a third switching unit for applying the third voltage to the electromechanical transducer; and a controller for controlling the first switching unit, the second switching unit, the third switching unit and the fourth switching unit so that the drive voltage repeats the cycle.
In the embodiment, it is possible to employ common parts for the switching units.
As an embodiment, the electromechanical transducer having a pair of terminals, and wherein the drive circuit comprises: a first switching element operating as a switch and having a pair of terminals; a second switching element operating as a switch and having a pair of terminals; a third switching element operating as a switch and having a pair of terminals; a fourth switching element operating as a switch and having a pair of terminals; and a controller for controlling the first switching element, the second switching element, the third switching element and the fourth switching element, wherein one of the terminals of the electromechanical transducer is connected to one of the terminals of the first switching element and one of the terminals of the second element, wherein the other of the terminals of the electromechanical transducer is connected to one of the terminals of the third switching element and one of the terminals of the fourth element, wherein the other of the terminals of the first switching element and the other of the terminals of the third switching element are connected to one of terminals of a power source, and wherein the other of the terminals of the second switching element and the other of the terminals of the fourth switching element are connected to the other of the terminals of the power source.
In order to achieve another of the above objects, according to another aspect of the present invention, there is provided a drive method for running a drive mechanism which comprises: an electromechanical transducer having a pair of ends in expanding and contracting direction; a drive member fixed to one of the ends of the electromechanical transducer; and a driven member which contacts frictionally with the drive member under a predetermined frictional force exerted therebetween, the drive method comprising: a first step of generating a drive voltage, wherein the drive voltage repeats a cycle of a first voltage having a value which is one of a maximum value and a minimum value, a second voltage having a value which is a value intermediate between the maximum value and the minimum value, and a third voltage having a value which is the other of the maximum value and the minimum value; a second step of applying the drive voltage generated at the first step to the electromechanical transducer; and a third step of expanding and contracting the electromechanical transducer by the drive voltage applied to the electromechanical transducer at the second step so as to move the driven member along the drive member relatively.
Generally, the electromechanical transducer (for example, piezoelectric transducer, electrostriction transducer, magnetostriction transducer, electrostatic actuator and the like) changes the electrical energy (for example, electric voltage, electric current, electric field, magnetic field, static electricity and the like) supplied thereto into the mechanical energy (for example, transformation or strain such as prolonging, compressing, expanding, contracting, bending, twisting and the like).
In the configuration, while the drive voltage changes from the first voltage to the third voltage through the second voltage, the change of the drive voltage is relatively gradual. Therefore, the electromechanical transducer expands or contracts gradually so that the driven member moves together with the drive member without sliding with respect to the drive member substantially. In the meantime, while the drive voltage changes from the third voltage to the first voltage with maximum amplitude thereof, the change of the drive voltage is relatively rapid. Therefore, the electromechanical transducer expands or contracts rapidly so that the driven member slides with respect to the drive member. By repeating such a cycle, it is possible to move or drive the driven member with respect to the drive member in a predetermined direction.
According to the drive method, even if a period for which the second voltage is applied to the electromechanical element varies, variation of sliding amount per cycle caused by changing the drive voltage from the third voltage to the first voltage is little or nothing, and only the frequency of sliding, or interval period of the cycle, changes. Thereby, it is possible to run the drive mechanism at a low speed stably, by extending the period for which the second voltage is applied to the electromechanical transducer. Moreover, it is possible to realize the drive circuit for switching the drive voltage as descried above, employing a simple construction.
As an embodiment, inequalities of
(n1+1)Txe2x88x920.3Txe2x89xa6taxe2x89xa6(n1+1)T+0.3Txe2x80x83xe2x80x83(1xe2x80x2)
0.95(n2+0.5)Txe2x88x920.3Txe2x89xa6tbxe2x89xa60.95(n2+0.5)T+0.3Txe2x80x83xe2x80x83(2xe2x80x2)
ta+tb less than Tdxe2x80x83xe2x80x83(3xe2x80x2)
are satisfied, where T is a resonance period of a system including the electromechanical transducer and the drive member, where Td is a drive period of the drive voltage applied to the electromechanical transducer by the drive circuit, where ta is a first period for which the first voltage is applied to the electromechanical element, where tb is a third period for which the third voltage is applied to the electromechanical transducer, where n1 is one of zero and positive integers, and where n2 is one of zero and positive integers.
When the first period for which the first voltage is applied to the electromechanical element and the third period for which the third voltage is applied to the electromechanical transducer have some relationship with the resonance period of the system including the electromechanical transducer and the drive member as described in the above embodiment, it is possible to run the drive mechanism efficiently.
As an embodiment, the first voltage and the third voltage are equal and of opposite sign, and wherein the second voltage is zero volt.
In the embodiment, it is possible to reduce variation or kinds of parts of the drive circuit in order to employ a simple construction.
As an embodiment, applying the drive voltage to the electromechanical transducer at the third step causes the electromechanical transducer to expand at a first velocity and to contract at a second velocity, different from the first velocity, so as to move the driven member along the drive member relatively.
As an embodiment, the drive voltage generated at the first step has a generally square waveform comprising horizontal straight parts and vertical straight parts.